The present invention relates to a magnetic recording medium, and in particular to a magnetic recording medium with improved electromagnetic characteristics and durability.
In recent years, the data recorded on magnetic recording medium has been increasingly digitalized to prevent deterioration of signals due to repeated copying or deterioration of picture quality or sound quality.
However, it is inevitably necessary to record data at higher density because of the enormous increase in the amount of data to be recorded in digital VTR, for example, and it is necessary to record the data in shorter wavelength. For this purpose, metal magnetic powder having finer highly saturated magnetic flux density is used as the magnetic powder. When such metal magnetic powder is used, the dispersion property, coating material (paint) stability and coating material viscosity of the magnetic power exerts extensive influence on the properties of the recording medium.
Specifically, if the dispersion property of the magnetic powder is low, the surface property and magnetic characteristics of the medium are decreased, and this leads to deterioration of electromagnetic characteristics (output, C/N, etc.). Also when the balance between coating material viscosity and yield value is turned toward lower leveling (i.e. higher yield value), the surface property of the medium is worsened and this leads to the results as described above. This also causes lower working efficiency in the manufacture of the magnetic recording medium.
Further, if the stability of the coating material is poor, the dispersion property and viscosity in the initial stage may not be maintained throughout the production process, and this also leads to the results described above.
To overcome these problems, it has been considered to use a binder, which has a functional group exhibiting high adsorbing property by magnetic powder.
More concretely, as disclosed in Japanese Patent Publication Laid-Open 63-261523, Japanese Patent Publication 6-10860, etc., the use of a binder having a polar function group such as --COOM, --SO3M, --OSO3M, etc. has been proposed.
For example, the following magnetic recording media have been proposed: a magnetic recording medium having a magnetic layer with ferromagnetic fine powder evenly dispersed in a binder, said magnetic layer being disposed on a non-magnetic support member, whereby said magnetic recording medium comprises ferromagnetic fine powder having BET of 45 m2/g, coercive force of not less than 1000 Oe and acicular ratio of not less than 7, and a binder having a polar group of --COOM, --SO3X, --OSO3X, --PO3X2 or --OPO3X2 and consisting of a polymer having a molecular weight of 10,000 to 100,000, whereby the weight ratio of the ferromagnetic fine powder to the binder is 1.5 to 10 (disclosed in Japanese Patent Publication Laid-Open 63-261523); or a magnetic recording medium comprising a non-magnetic support member and a magnetic layer disposed on said support member, said magnetic layer contains polyurethane resin and vinyl chloride copolymer, ferromagnetic metal fine powder containing iron, cobalt, nickel or an alloy of these metals and an abrasive material are dispersed in a binder consisting of a resin component, at least one of said resins has at least one type of polar groups of --COOM, --SO.sub.3 M, --OSO.sub.3 M, and --PO(OM').sub.2 [where M represents a hydrogen atom or an alkali metal atom, and M' represents a hydrogen atom, an alkali metal or a lower hydrocarbon] whereby said ferromagnetic metal fine powder contains an aluminum component by 1 to 6 weight % in the weight converted to metal (disclosed in Japanese Patent Publication 6-10860).
Although the dispersion property is improved when a resin containing the above polar functional group is used as a dispersion agent, the balance between coating material stability or coating material viscosity and yield value is not sufficient, and the product is not very satisfactory on these points.
In this respect, there have been strong demands for a magnetic recording medium, by which it is possible to improve dispersion property, coating material stability, and coating material viscosity of the magnetic recording medium using metal magnetic powder, and to improve electromagnetic characteristics and productivity.
To solve the above problems, the prevent inventors have found as a result of fervent study that the above object can be attained by a binder which contains metal sulfamate having a functional group, resulting in the present invention.